Photonic crystals, the photonic analog of traditional semiconductors, are currently the subject of intense investigation for applications in integrated optics. Such crystals is exhibit a photonic bandgap (analogous to the electronic bandgap of semiconductors) that determines which photon wavelengths can propagate through the crystal. By introducing specific defects into the crystal's bandgap structure, the flow of photons inside the crystal can be manipulated and a variety of micrometer-scale optical devices fabricated.
In particular, it is postulated that a three-dimensional (3-D) photonic crystal with appropriate line defects will act as a low-loss waveguide device, enabling light to be turned through a 90-degree bend without significant scattering losses. Such a device would be potentially useful in increasing the density of components on an optical chip and in chip-to-back plane applications. Other potential device applications include use in optical interconnects, optical switches, couplers, isolators, multiplexers, tunable filters, and low-threshold emitters.
Various methods have been used to produce photonic crystals. Such methods include, for example, colloidal crystal self-assembly methods, self-assembly of block copolymers, semiconductor-based lithography (for example, photolithography, masking, and etching), mechanical production of an array of holes in a substrate material (or, alternatively, mechanical removal of substrate material to form a periodic pattern of cylindrical or similarly-shaped rods of substrate material), electrochemical production of pore arrays in a substrate material, photofabrication using multibeam interference (MBI) or holographic lithography techniques, glancing angle deposition, and multi-photon photofabrication. Each method has its own advantages. Generally, however, all but MBI suffer from one or more of the following disadvantages: relatively high inherent structural disorder or defect concentrations, relatively slow deposition speeds, and numerous requisite process steps.
In contrast, MBI techniques enable the production of photonic crystals of various different Bravais lattice structures at relatively high speeds and using only a few process steps. The resulting photonic crystals typically exhibit exceptional order and structural fidelity.
Generally, organic photoresists have been used for MBI, but most conventional organic photopolymers have refractive indices that are substantially less than two and lack sufficient thermal stability to remain intact during, for example, the chemical vapor deposition (CVD) process that is often used to deposit high refractive index semiconductor (for silicon, typically temperatures above 500° C.). Such deficiencies make it difficult or impossible to achieve the high refractive index contrast ratios capable of supporting a complete photonic bandgap.